This invention relates to the structure of a semiconductor laser array device.
There have been many researches to develop a high-output semiconductor laser array device with a plurality of optically connected semiconductor lasers disposed on a single substrate. Since the light-emitting area of a semiconductor laser array may be as wide as several .mu.m to several hundred .mu.m, temperature of the active region of the semiconductor lasers tends to become higher near the center of such a light-emitting area of an array than at its peripheral regions by several degrees when the device is operating at a high output rate. This causes the wavelength of laser oscillations or the constant of propagation to become different between the center and peripheral regions of the light-emitting area. In other words, the oscillations are no longer in phase or in synchronism over the entire light-emitting area of the array.
Before the distinguishing characteristics of the present invention are explained, formation of a semiconductor laser array device according to a previously considered design is described below by way of FIGS. 8A and 8B. As shown more clearly in FIG. 8A, a current-narrowing layer 82 of n-type GaAs (or GaAlAs) is formed by a liquid phase epitaxial growth method on a p-type GaAs substrate 81 and grooves 83 penetrating the n-type GaAs (or GaAlAs) are thereafter formed by photolithography and etching technologies on the substrate surface. FIG. 8B shows the grooves 83 thus formed as seen from above. Next, a p-Ga.sub.1-x Al.sub.x As cladding layer 83, a p-(or n-) Ga.sub.1-y Al.sub.y As active layer 84, an n-type Ga.sub.1-x Al.sub.x As cladding layer 85 and an n.sup.+ -GaAs capping layer 86 are sequentially formed by liquid phase epitaxial growth method, where 0.ltoreq.y.ltoreq.x.ltoreq.1. After electrodes 87 and 88 are formed on both sides and resonance surfaces are formed by cleaving, individual laser array elements are partitioned. Thereafter, it is brazed onto a heat sink 89 of copper, diamond or the like by using a soldering material 90 such as indium or a gold-tin alloy such that the heat generated near the active layer 84 at the time of laser oscillations can be effectively removed. A gold alloy layer 91 is formed on the heat sink 89 in order to improve its current flow characteristics and contact with the brazing material.
When a phase-synchronized laser array device thus formed and having a wide light-emitting area is operated under a condition of high optical output, the temperature distribution in the vicinity of the active layer 84 becomes as shown in FIG. 8C, temperature being higher near the center and lower at peripheral regions. The temperature difference can be as large as several degrees. The temperature-dependence of oscillating wavelength of a semiconductor laser is on the order of about 1.ANG./.degree.C. and mutually adjacent laser light-emitting areas cannot oscillate under a phase-synchronized condition if the difference in oscillation wavelength between them becomes about 0.3.ANG. or greater. As a result, oscillations cannot be obtained from the entire light-emitting area in one axial mode nor in a single transverse mode. Thus, it becomes difficult to use a lens to focus the beams to the diffraction limit and the optical output becomes unstable because of the transverse modes which are mixed in the oscillations. Moreover, there will be more noise in the optical output and the operation lifetime will become shorter.